The invention relates to ceramic compositions having high effective dielectric constants for use in capacitors. More particularly, the invention relates to dielectric bodies of the intergranular insulation type and to methods of producing such dielectric bodies. A large effective dielectric constant is achieved in this type of structure by electrically insulating the grain boundaries of a conventional ceramic semiconductor with a material having a high dielectric constant.
The invention also relates to the production of semiconductive dielectric bodies, having electrically insulating dielectric materials at their grain boundaries, by coating the body with the insulating material and heating the coated body to diffuse the insulating material along the grain boundaries.
U.S. Pat. No. 3,933,668 discloses intergranular insulation type ceramic semiconductor compositions comprising a major portion of SrTiO.sub.3, minor amounts of Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5, and minor amounts of GeO.sub.2 or ZrO. The ceramic has either (i) Bi.sub.2 O.sub.3 or (ii) Bi.sub.2 O.sub.3, PbO, and B.sub.2 O.sub.3 diffused therein at the grain boundaries. The first ingredients are mixed, shaped into discs, and fired in a reducing atmosphere at, for example, 1350.degree. C. to 1480.degree. C. The fired discs are then coated with Bi.sub.2 O.sub.3, alone or in combination with PbO and B.sub.2 O.sub.3, and then sintered at, for example, 1300.degree. C. for approximately two hours in an oxidizing atmosphere to diffuse the coating into the grain boundaries.